Fig. 64. Ceterach officinarum.
It is well known that the seeds of tropical plants have been brought to the western coasts of Scotland and Ireland by the great Atlantic currents; it might therefore be expected, that the spores of Cryptogams brought to our shores by the same means, and finding a congenial climate, should become naturalized, as appears to have been the case in ancient times, with regard to the Ceterach and Asplenium marinum, from their migrations, as traced by Mr. Johnson. ‘The countries bordering on the basin of the Mediterranean and the islands and eastern shores of the North Atlantic appear to have been the original stations of this remarkable fern. In the British islands its distribution is too partial to admit of its being regarded as strictly indigenous, though probably naturalized here at a period little subsequent to the arrival of Asplenium marinum. It occurs here on limestone-rocks, but more frequently on old walls and ruins, rooted deeply in the decaying mortar, and often accompanying Asplenium Ruta-muraria and Asplenium Trichomanes. Like other natural importations from the south, it is found most abundantly on the western maritime counties that receive the more direct flow of the tide, and has progressed slowly towards the northern and central parts of the kingdom. In Scotland it has not yet traversed beyond Perth, and is still regarded as a rare species; while in Ireland its copious distribution seems to indicate an earlier arrival.’
The group Vittarieæ, consisting mainly of Vittaria and Tæniopsis, is essentially equatorial or sub-equatorial. The plants have narrow ribbon-like fronds, with naked sori immersed in the very margin of the frond, in a more or less sunken furrow, there being no indusium.
The Blechnum Spicant, which may be taken as a type of the Lomarieæ, is a very ornamental fern, with its dark green, linear-lanceolate fronds, of two forms; the fertile ones erect, pectinate, pinnate, with distinct narrow linear and acute pinnæ, while the barren ones are smooth, spreading, and pinnatifid, with broad linear, blunt, approximate lobes. The rachis is generally smooth, and of a dark purple hue, its leafless portion being shaggy with membranaceous scales. The barren fronds lie on the ground in the winter, while the fertile ones are erect, bear fruit from May to October, and wither when they have shed their spores. In the fertile fronds the lateral veins are alternate, and extend obliquely upwards, about half way towards the margin of the lobe, when, by a sudden turn, each runs parallel to the mid-vein, and anastomoses with the one above it, thus forming an apparently longitudinal vein on which the sorus is placed, so as to form a line on each side of the mid-vein; this is covered with a continuous indusium like a hem, which opens on the interior side ([fig. 65]). In the barren fronds the veins do not anastomose at the margins of the lobe. This species is common in almost every part of Great Britain, and extends on the Continent from Swedish Lapland to the borders of the Mediterranean.
The species of Lomaria are marked by having the fertile fronds contracted, so that the sorus is quite marginal; but there are many species of Blechnum in which the elongated sorus, placed parallel to the midrib, is quite distant from the margin, much more so than in Blechnum Spicant.
Fig. 65. Blechnum Spicant.
Fig. 66. Pteris aquilina.